Thin film transistor

ABSTRACT

A method for manufacturing a semiconductor device such as a thin film transistor using a crystal silicon film is provided. The crystal silicon film is obtained by selectively forming films, particles or clusters containing nickel, iron, cobalt, ruthenium, rhodium, paradium, osmium, iridium, platinum, scandium, titanium, vanadium, chrome, manganese, copper, zinc, gold, silver or silicide thereof in a form of island, line, stripe, dot or film on or under an amorphous silicon film and using them as a starting point, by advancing its crystallization by annealing at a temperature lower than a normal crystallization temperature of an amorphous silicon. A transistor whose leak current is low and a transistor in which a mobility is high are obtained in the same time in structuring a dynamic circuit having a thin film transistor by selectively forming a cover film on a semiconductor layer which is to become an active layer of the transistor and by thermally crystallizing it thereafter.

This is a Divisional application of Ser. No. 08/160,908, filed Dec. 3,1993 now U.S. Pat. No. 5,403,772.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating an integratedcircuit or more concretely to a semiconductor circuit having a matrixdevice (including an electro-optic display and semiconductor memory)having a matrix structure and a MOS or MIS(metal-insulator-semiconductor) type field effect element (hereinaftergenerally referred to as a MOS type element) as a switching element andcharacterized in its dynamic operation such as a liquid crystal displaydevice and dynamic RAM (DRAM) and a driving circuit therefor or anintegrated driving circuit like an image sensor. The present inventionparticularly relates to a device employing a thin film semiconductorelement such as a thin film semiconductor transistor or the like whichis formed on an insulating surface as a MOS type element and to a devicehaving a thin film transistor whose active layer is formed by crystalsilicon.

2. Description of the Related Art

Conventionally, a crystalline silicon semiconductor thin film used for athin film device such as a thin film insulated gate type field effecttransistor (TFT) has been fabricated by crystallizing an amorphoussilicon film formed by a plasma CVD or thermal CVD method within such anapparatus as an electric furnace for many hours of more than 24 hours ata temperature more than 600° C. The many hours of heat treatment hasbeen required in order to obtain sufficient characteristics such as ahigh field mobility and high reliability.

However, such conventional method has many problems. One of the problemsis that its throughput is low and accordingly, a product cost becomeshigh. For example, if it takes 24 hours for the crystallization processand if it takes two minutes of processing time per sheet of substrate,720 substrates must be processed in the same time. However, a tubefurnace normally used can process 50 sheets of substrates at most in onetime, and when only one apparatus (reaction tube) is used, it takes 30minutes per sheet. That is, in order to process one sheet in 2 minutes,15 reaction tubes must be used. It means that a scale of investment mustbe increased and that because the investment is greatly depreciated, itcannot but be reflected in the product cost.

Another problem lies in the temperature of the heat treatment. Normally,substrates used for fabricating a TFT are roughly divided into thosecomposed of pure silicon oxide such as silica glass and non-alkalineboro-silicated glass such as Corning No. 7059 (hereinafter referred toas Corning 7059). Among them, the former has no problem in terms oftemperature because it has an excellent heat resistance and can behandled in the same manner with the wafer process of normalsemiconductor integrated circuits. However its cost is high andincreases exponentially as the substrate area increases. Accordingly, itis used only for TFT integrated circuits having a relatively small area.

On the other hand, non-alkaline glass has a problem in terms of heatresistance, though its cost is sufficiently low as compare to that ofsilica glass. Because its strain point is generally around 550°to 650°C., or less than 600° C. in case of a readily available material, suchproblems as irreversible shrinkage and warp are caused on the substratein a heat treatment at 600° C. and it is remarkable in such a substratewhose diagonal distance exceeds 10 cm. From above reasons, it has beenconsidered to be indispensable to keep the heat treatment conditionsunder 550° C. and within 4 hours to reduce the cost in crystallizingsilicon semiconductor films. It is then an object of the presentinvention to provide a semiconductor fabricating method that clears suchconditions and a semiconductor device fabricating method using such asemiconductor.

Lately, a study on an insulated gate type semiconductor device having athin film active layer (or called as an active region) has beenconducted. Especially, a thin film insulated gate transistor orso-called a thin film transistor (TFT) has been fervently studied. Theyare formed on a transparent insulating substrate to use to control eachpicture element and to drive its matrix in a display device such as aliquid crystal display having a matrix structure or to use as a drivingcircuit of an image sensor formed similarly on an insulating substrate.They are categorized as an amorphous silicon TFT or crystalline silicon(or called as polycrystalline silicon) TFT depending on a material andcrystal state of a semiconductor used.

Lately, a study to use a material which presents an intermediate statebetween crystalline silicon and amorphous is also being conducted.Although the intermediate state is being discussed, all those whichreached to some crystal state by any thermal process (such as byannealing at a temperature more than 450° C. by irradiating strongenergy such as laser light) shall be called as crystalline silicon inthis specification.

A crystalline silicon TFT is used also in a monocrystal siliconintegrated circuit as a so-called SOI technology and it is used as aload transistor for example in a highly integrated SRAM. In this case,however, an amorphous silicon TFT is rarely used.

Further, a very high speed operation is possible in a semiconductorcircuit on an insulating substrate because there is no capacitivecoupling between the substrate and wires, so that a technology to use itas a very high speed microprocessor or very high speed memory is beingproposed.

Generally, a field mobility of a semiconductor in an amorphous state issmall and accordingly, it cannot be used for a TFT requiring a highspeed operation. Furthermore, because a field mobility of P-type isremarkably small in an amorphous silicon, a P-channel type TFT (a TFT ofPMOS) cannot be fabricated and accordingly, a complementary NOS circuit(CMOS) cannot be formed by combining with a N-channel type TFT (a TFT ofNMOS).

However, a TFT formed by an amorphous semiconductor has an advantagethat OFF current is small. Then it is utilized in the use in which avery high speed operation is not required, only one conductive type willdo and a TFT having a high charge retaining ability is required such astransistors of an active matrix of a liquid crystal display having asmall matrix scale. However, it has been difficult to use the amorphoussilicon TFT for an advanced application such as a liquid crystal displayhaving a large scale matrix. Further, it could not be used naturally forperipheral circuits of a display and for a driving circuit of an imagesensor which require a high speed operation.

On the other hand, a crystalline semiconductor has a field mobilitylarger than that of the amorphous semiconductor and accordingly, a highspeed operation is possible. For example, such a large value as 300 cm²/Vs has been obtained as a field mobility in a TFT using a silicon filmre-crystallized by laser annealing. It is an extremely large valueconsidering that a field mobility of a NOS transistor formed on a normalmonocrystal silicon substrate is around 500 cm² /Vs. Whereas anoperation speed of the NOS circuit on the monocrystal silicon is limitedby a parasitic capacity between the substrate and wires, there is nosuch limit in terms of the TFT using crystallized silicon film becauseit is formed on the insulating substrate. Accordingly, a remarkable highspeed operation is being expected to be achieved in such TFT.

Further, it is possible to form a CMOS circuit by the crystallinesilicon because not only a NMOS TFT but also a PMOS TFT can be similarlyobtained. For example, among liquid crystal displays in an active matrixsystem, one having a so-called monolithic structure in which not onlythe active matrix section but also peripheral circuits (such as adriver) are constructed by a CMOS crystalline silicon TFT is known. TheTFT used in the aforementioned SRAM is what this point is noticed,wherein the PMOS is constructed by the TFT as a load transistor.

Furthermore, whereas it is difficult to form a source/drain region bysuch a self-aligning process as those used in monocrystal IC technologyin a normal amorphous TFT and a parasitic capacity caused by thegeometrical overlap of a gate electrode and the source/drain regioncauses a problem, the crystalline silicon TFT has such an advantage thata parasitic capacity can be remarkably suppressed because theself-aligning process can be adopted.

However, a leak current of the crystalline silicon TFT when no voltageis applied to the gate (non-selection time) is great as compare to thatof the amorphous silicon TFT, and such countermeasures have been takenthat an auxiliary capacity is provided to compensate the leak currentand that two TFTs are connected in series to reduce the leak currentwhen it is used in a liquid crystal display.

For example, a method to form an amorphous silicon and to irradiatelaser selectively on it to crystallize only a peripheral circuit hasbeen proposed for forming the peripheral circuit of polysilicon TFTshaving a high mobility monolithically on the same substrate whileutilizing a high OFF resistance of the amorphous silicon TFT.

Presently, however, its yield is low due to a problem of reliability ofthe laser irradiating process (such as a bad homogeneity of theirradiated energy within the irradiated surface), so that a method toconstruct a matrix by amorphous silicon TFTs and to construct drivingcircuits by connecting monocrystal integrated circuits by a TAB methodor the like is being adopted in the end. However, this method requires amore than 0.1 mm of pixel pitch from the physical restriction in theconnection and its cost becomes high.

The present invention is intended to solve such difficult problems, butit is not desirable to complicate the process and to lower its yield orto increase its cost for that end. What the present invention intends isto fabricate two kinds of TFTs, i.e. a TFT in which a high mobility isrequired and a TFT in which a leak current is required to be low,readily and discriminately while keeping mass-producibility and whileminimizing changes of its process.

Further, it is another object of the present invention to reduce adifference between the mobility of the NMOS and of the PMOS in the CMOScircuit. The reduced difference between the NMOS and PMOS allows toincrease a degree of freedom in designing the circuit.

Semiconductor circuits to which the present invention is applied are notuniversal. That is, the present invention is suited for an active matrixcircuit for displaying images by utilizing such materials which changetransmittivity or reflectivity of light by an effect of electric field,by sandwiching those materials between electrodes facing to each otherand by applying the electric field between those electrodes such as aliquid crystal display; a memory device for holding memory by storingcharge in capacitors such as DRAM; a circuit having a dynamic circuitsuch as a dynamic shift register which drives the next circuit bycapacitors of MOS structure portion of MOS transistors or othercapacitors; and a circuit having a digital circuit and a circuit forcontrolling analog signal outputs such as a driving circuit of an imagesensor. The present invention is suited especially to a circuit in whichdynamic circuits and static circuits are mixedly mounted.

SUMMARY OF THE INVENTION

The present invention is characterized in that a crystalline siliconfilm is obtained by forming island film, dots, particles, clusters orlines containing a material selected from the group consisting ofnickel, iron, cobalt, ruthenium, rhodium, paradium, osmium, iridium,platinum, scandium, titanium, vanadium, chrome, manganese, copper, zinc,gold and silver and a combination thereof on or under the silicon filmwhich is in an amorphous state or in a random crystal state (e.g. astate in which portions having a good crystallinity and amorphousportions are mixed) which can be said substantially as being in anamorphous state, and by annealing it at a temperature lower than acrystallization temperature in a mere heat treatment of normal amorphoussilicon for a shorter time. This annealing may be carried out in ahydrogen, oxygen or nitrogen atmosphere. This annealing can be carriedout by (1) heating for A hours in an atmosphere containing oxygen andthen heating for B hours in an atmosphere containing hydrogen: (2)heating for C hours in an atmosphere containing oxygen and then heatingfor D hours in an atmosphere containing nitrogen: (3) heating for Ehours in an atmosphere containing hydrogen and then heating for F hoursin an atmosphere containing oxygen: (4) heating for G hours in anatmosphere containing hydrogen and then heating for H hours in anatmosphere containing nitrogen: (5) heating for I hours in an atmospherecontaining nitrogen and then heating for J hours in an atmospherecontaining oxygen: (6) heating for K hours in an atmosphere containingnitrogen and then heating for L hours in an atmosphere containinghydrogen: (7) heating for M hours in an atmosphere containing oxygen,heating for N hours in an atmosphere containing hydrogen and thenheating for P hours in an atmosphere containing nitrogen: (8) heatingfor Q hours in an atmosphere containing oxygen, heating for R hours inan atmosphere containing nitrogen and then heating for S hours in anatmosphere containing hydrogen: (9) heating for T hours in an atmospherecontaining hydrogen, heating for U hours in an atmosphere containingoxygen and then heating for V hours in an atmosphere containingnitrogen: (10) heating for W hours in an atmosphere containing hydrogen,heating for X hours in an atmosphere containing nitrogen and thenheating for Y hours in an atmosphere containing oxygen: (11) heating forZ hours in an atmosphere containing nitrogen, heating for A' hours in anatmosphere containing oxygen and then heating for B' hours in anatmosphere containing hydrogen: or (12) heating for C' hours in anatmosphere containing nitrogen, heating for D' hours in an atmospherecontaining hydrogen and then heating for E' hours in an atmospherecontaining oxygen.

Concerning to the crystallization of silicon film, a method to form acrystalline island film as a nucleus or seed crystal and to grow itepitaxially in solid phase (for example Japanese Patent Laid-Open No.1-214110) has been proposed in the past. However, crystal barely grewunder 600° C. of temperature by such method. Generally silicon undergoesa process when it transfers from an amorphous state to a crystal statethat molecular chains in the amorphous state are parted and afterputting the parted molecules into a state that they would not couplewith other molecules again, molecules are recombined into a portion of acrystal in combination with some crystalline molecules. However, energyfor parting the initial molecular chains and for keeping them in thestate not to couple with other molecules is great in this process and ithas been blocking the crystallization reaction. In order to supply thisenergy, it takes several minutes with about 1000° C. of temperature orseveral tens of hours with about 600° C. of temperature. Because thetime exponentially depends on the temperature (=energy), an advancementof the crystallization reaction could not be observed almost at all atless than 600° C. or at 550° C. for example. The concept of epitaxialcrystallization in solid phase also could not give any solution to thisproblem.

Then the inventor of the present invention thought of lowering theblocking energy in the aforementioned process by some catalytic actionwhich is totally different from the concept of conventional solid phasecrystallization. The inventor noticed that nickel (Ni), iron (Fe),cobalt (Co), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os),iridium (Ir), platinum (Pt), scandium (Sc), titanium(Ti), vanadium (V),chromium (Cr), manganese (Mn), copper (Cu), zinc (Zn), gold (Au) andsilver (Ag) readily couple with silicon.

For example, the inventor noticed on that in a case of nickel, itreadily turns out to be nickel silicide (NiSi_(x), 0.4≦×≦2.5) and alattice constant of nickel silicide is close to that of silicon crystal.Then, when energy and other were simulated in a ternary system ofcrystal silicon--nickel silicide--amorphous silicon, it was found thatamorphous silicon readily reacts at the boundary with the nickelsilicide and that the following reaction is brought about:

amorphous silicon (silicon A) + nickel silicide (silicon B) → nickelsilicide (silicon A) + crystal silicon (silicon B) (silicon A and Bindicate positions of silicon)

A potential of the block of this reaction is fully low and a reactiontemperature is also low. This reaction formula indicates that thereaction proceeds while converting amorphous silicon into crystalsilicon by nickel. It was found that the reaction actually started atless than 580° C. and that the reaction could be observed even at 450°C. Though it is a matter of course, the higher the temperature, thefaster the speed of the advancement of reaction was. The same effect wasalso recognized with other metal elements described above.

According to the present invention, a crystal silicon region is expandedby forming a film, particle or cluster containing at least one of Ni,Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Sc, Ti, V, Cr, Mn, Cu, Zn, Au and Agsuch as island, stripe, line, dot or film of nickel or other simplemetal substances described above or their silicide as a starting pointand by developing those metal elements from the point to surroundingsalong the reaction described above. By the way., oxide is not preferableas a material containing those metal elements, because oxide is a stablecompound and cannot start the aforementioned reaction.

The crystal silicon expanded from a specific point has a structure closeto monocrystal with good continuity of crystallinity, though it differsfrom the conventional solid phase epitaxial growth, so that it isconvenient in using for semiconductor devices such as a TFT. However, aninfinite number of crystallization starting points existed when amaterial containing the aforementioned metals for accelerating thecrystallization such as nickel was provided homogeneously on a substrateand due to that, it was difficult to obtain a film having a goodcrystallinity.

A better result was obtained when a concentration of hydrogen was lessin the amorphous silicon film as the starting material of thiscrystallization. However, because hydrogen was released as thecrystallization advanced, no such a clear correlation was seen between aconcentration of hydrogen within a silicon film obtained and that in theamorphous silicon film as the starting material. The concentration ofhydrogen within the crystal silicon of the present invention wastypically more than 0.01 atomic percent and less than 5 atomic percent.

Although Ni, Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Sc, Ti, V, Cr, Mn, Cu, Zn,Au and Ag are used in the present invention, these materials are notgenerally preferable for silicon as a semiconductor material and it isnecessary to remove them. In terms of nickel, because nickel silicidewhich reached the end of the crystallization as a result of theaforementioned reaction is easily resolved in hydrofluoric acid orhydrochloric acid or dilution of them, nickel can be reduced from thesubstrate by means of a treatment by those acids. Further, those metalelements can be positively reduced by treating at 400 °to 600° C. in anatmosphere containing chlorine such as hydrogen chloride, variousmethane chlorides (CH₃ Cl, CH₂ Cl₂, CHCl₃), various ethane chlorides (C₂H₅ Cl, C₂ H₄ Cl₂, C₂ H₃ Cl₃, C₂ H₂ Cl₄, C₂ HCl₅) or various ethylenechlorides (C₂ H₃ Cl, C₂ H₂ Cl₂, C₂ HCl₃) after the end of thecrystallization process. Especially, trichloroethylene (C₂ HCl₃) is amaterial which can be easily used. A concentration of Ni, Fe, Co, Ru,Rh, Pd, Os, Ir, Pt, Sc, Ti, V, Cr, Mn, Cu, Zn, Au and Ag in the siliconfilm of the present invention was typically more than 0.005 atomicpercent and less than 1 atomic percent.

In using the crystal silicon film fabricated according to the presentinvention for a semiconductor element such as a TFT, it is notpreferable to provide the semiconductor element on the end of thecrystallization (it is also a portion where the crystallization startedfrom a plurality of starting points hit each other) because a largegrain boundary (a portion where crystallinity is discontinued) existsand the concentration of metal elements which accelerate thecrystallization such as nickel is high there as it is obvious from thedescription above. Accordingly, a pattern of a coating film containingthe metal elements which become the starting point of thecrystallization and accelerate it such as nickel and a pattern of thesemiconductor element must be optimized in forming the semiconductorelement utilizing the present invention.

In the present invention, roughly there are two methods for patterningthe metal elements which accelerate the crystallization. A first methodis to selectively form a film or the like of those metals before formingan amorphous silicon film and a second method is to selectively form afilm or the like of those metals after forming the amorphous siliconfilm.

The first method can be implemented by using a normal photolithographicmeans or lift-off means. The second method is complicated more or less.That is, if the film or the like of the metals which accelerate thecrystallization is formed adhering to the amorphous silicon film, themetal and the amorphous silicon partially react each other producingsilicide when forming the film. Therefore, it is necessary to fully etchsuch silicide layer when patterning after forming the metal film or thelike.

In the second method, the lift-off method is relatively easily carriedout. In this case, organic materials such as photoresist or non-organicmaterials such as silicon oxide or silicon nitride may be used as amasking material. A processing temperature must be taken intoconsideration in selecting the masking material. Further, because amasking action differs depending on materials, a full attention needs tobe paid on it. Especially, a film of silicon oxide or silicon nitrideformed by various CVD methods has many pinholes and the crystallizationmay advance from an unexpected section if the film is not fully thick.

Generally, patterning is implemented after forming the coating filmusing those masking materials to selectively expose the surface ofamorphous silicon. Then the metal film or the like which accelerates thecrystallization is formed.

What must be taken care of in the present invention is the concentrationof metal elements within the silicon film. Although nothing is betterthan that its amount is small, it is also important for the amount to bealways kept constant. It is because a considerable fluctuation in thedegree of crystallization is brought about per lot in the manufacturingsite if the amount of the metal element fluctuates significantly. Itbecomes more difficult to reduce the fluctuation of the amountespecially when the amount of the metal element is required to be less.

In the first method, because the metal film or the like selectivelyformed is coated by the amorphous silicon film, it cannot be taken outlater to adjust its amount. In terms of the amount of metal elementrequired in the present invention, the thickness of the metal film orthe like is so thin as several to several tens angstrom, so that it isdifficult to form the film with a good reproducibility.

The same also applies to the second method. However, there is a room ofimprovement in the second method as compare to the first method becausethe metal film or the like which accelerate the crystallization existson the surface in this method. That is, a fully thick metal film isformed and a heat treatment (pre-annealing) is implemented at atemperature lower than an annealing temperature before annealing toreact a part of the amorphous silicon film and the metal film and toproduce silicide. Then the metal film which did not react is etched.Although it depends on a metal used, there is no problem particularly interms of Ni, Fe, Co, Ti and Cr because there is an etchant in which anetching rate of the metal film and the silicide is fully large.

In this case, a thickness of the silicide layer obtained is determinedby the temperature and time of the heat treatment (pre-annealing) andthe thickness of the metal layer has almost nothing to do with it. Dueto that, the very small amount of metal element introduced in theamorphous silicon film can be controlled.

The present invention also utilizes that there is a difference indegrees of crystallization when the surface of semiconductor is coveredby a coating film (cover film) of silicon oxide or silicon nitride andwhen not covered when crystallizing a crystalline silicon TFT in atemperature at 450° to 1000° C. or preferably at 500° to 800° C. in anatmosphere containing oxygen, hydrogen or nitrogen. Such atmosphere canbe an atmosphere containing oxygen, atmosphere containing hydrogen,atmosphere containing nitrogen, atmosphere containing oxygen andhydrogen, atmosphere containing oxygen and nitrogen, atmospherecontaining hydrogen and nitrogen or atmosphere containing oxygen,hydrogen and nitrogen. The aforementioned crystallization can be carriedout by (1) heating for A hours in the atmosphere containing oxygen andthen heating for B hours in the atmosphere containing hydrogen: (2)heating for C hours in the atmosphere containing oxygen and then heatingfor D hours in the atmosphere containing nitrogen: (3) heating for Ehours in the atmosphere containing hydrogen and then heating for F hoursin the atmosphere containing oxygen: (4) heating for G hours in theatmosphere containing hydrogen and then heating for H hours in theatmosphere containing nitrogen: (5) heating for I hours in theatmosphere containing nitrogen and then heating for J hours in theatmosphere containing oxygen: (6) heating for K hours in the atmospherecontaining nitrogen and then heating for L hours in the atmospherecontaining hydrogen: (7) heating for M hours in the atmospherecontaining oxygen, heating for N hours in the atmosphere containinghydrogen and then heating for P hours in the atmosphere containingnitrogen: (8) heating for Q hours in the atmosphere containing oxygen,heating for R hours in the atmosphere containing nitrogen and thenheating for S hours in the atmosphere containing hydrogen: (9) heatingfor T hours in the atmosphere containing hydrogen, heating for U hoursin the atmosphere containing oxygen and then heating for V hours in theatmosphere containing nitrogen: (10) heating for W hours in theatmosphere containing hydrogen, heating for X hours in the atmospherecontaining nitrogen and then heating for Y hours in the atmospherecontaining oxygen: (11) heating for Z hours in the atmosphere containingnitrogen, heating for A' hours in the atmosphere containing oxygen andthen heating for B' hours in the atmosphere containing hydrogen: or (12)heating for C' hours in the atmosphere containing nitrogen, heating forD' hours in the atmosphere containing hydrogen and then heating for E'hours in the atmosphere containing oxygen. It is particularly preferable(4) to heat for G hours in the atmosphere containing hydrogen and thento heat for H hours in the atmosphere containing nitrogen, (5) to heatfor I hours (4 hours for example) in the atmosphere containing nitrogenand then to heat for J hours (1 hour for example) in the atmospherecontaining oxygen, or (6) to heat for K hours (4 hours for example) inthe atmosphere containing nitrogen and then to heat for L hours (1 hourfor example) in the atmosphere containing hydrogen. Generally thecrystallinity is good and as a natural consequence, a TFT having a highmobility can be obtained when the cover film exists. However, generallyits leak current becomes significant. On the other hand, the one havingno cover film has an advantage that the leak current is low, though thecrystallinity is not good and its mobility is low because it turns outbe amorphous state depending on temperature.

This characteristic is considered to be governed by the existence ofhydrogen, oxygen or nitrogen within the atmosphere infiltrated to theactive layer. This crystallization may be carried out by implementing itin nitrogen for example and then in hydrogen or oxygen. Thus TFTs havingdifferent characteristics may be formed on the same substrate in thesame time and in the same process. For example, the former high mobilityTFT can be used as a driving circuit of a matrix and the latter low leakcurrent TFTs can be used as TFTs in the matrix portion.

Or a mobility in the NMOS can be relatively reduced as compare to thatin the PMOS and a difference between the both can be almost eliminatedin the optimum condition by not providing the cover film on the NMOSregion and providing it on the PMOS region in the CMOS circuit.

The temperature of the thermal crystallization is an important parameterand the crystallinity of a TFT is determined by the temperature in thepresent invention. Generally, the temperature of thermal annealing isrestricted by a substrate and other materials. As far as the restrictionof a substrate material is concerned, a thermal annealing of up to 1100°C. is possible when silicon and silica are used as a substrate. However,it is desirable to anneal at less than 650° C. of temperature in a caseof Corning 7059 glass which is a typical non-alkaline glass. However, itmust be set considering characteristics required for each TFT, otherthan the substrate, in the present invention from the aforementionedreasons. When the annealing temperature is high, generally a growth ofcrystal of TFT advances, the mobility is increased and the leak currentincreases. Accordingly, the annealing temperature should be 450° to1000° C. or preferably 500° to 600° C. in order to obtain TFTs havingdifferent characteristics on the same substrate like the presentinvention.

One such example of the present invention is that in a display sectionof an active matrix circuit of a liquid crystal display or the like,polysilicon TFTs are used as switching transistors and that no coverfilm is provided in the active matrix region when crystallizing anactive layer and on the other hand the cover film is provided on theperipheral circuit region to turn the former into low leak current TFTsand the latter into high mobility TFTs.

FIG. 8(A) shows a conceptual drawing of a device having a displaycircuit section (active matrix) and a driving circuit (peripheralcircuit) therefor as described above. In the figure, a display device isshown in which a data driver 101 and gate driver 102 are arranged, anactive matrix 103 having TFTs is arranged in the middle and those driversections and the active matrix are connected through gate lines 105 anddata lines 108 on an insulating substrate 107. The active matrix 103 isan aggregate of picture element cells 104 having a NMOS or PMOS TFT(PMOS in the figure).

For a CMOS circuit of the driver section, a concentration of impuritiessuch as oxygen, nitrogen and carbon in the active layer is desirable tobe less than 10¹⁸ cm⁻³ or preferably less than 10¹⁷ cm⁻³ in order toobtain a high mobility. As a result, a threshold voltage of the TFT was0.5 to 2 V in the NMOS and -0.5 to -3 V in the PMOS for example and amobility was 30 to 150 cm² /Vs in the NMOS and 20 to 100 cm² /Vs in thePMOS.

On the other hand, an auxiliary capacity could be reduced and further betotally eliminated in the active matrix section by using single or aplurality of elements in series having as low as about 1 pA of leakcurrent with 1 V of drain voltage.

A second example of the present invention relates to a semiconductormemory. A semiconductor memory device by means of monocrystal ICs hasalready reached to its limit in terms of speed. Although it is necessaryto increase a current capacity of transistors in order to operate it ina higher speed, it causes a further increase of power consumption and itcannot but be dealt by increasing driving voltage so long as a capacityof capacitors cannot be increased further for a DRAM which carries outmemory operations by storing charge in the capacitors.

One reason why it is said that monocrystal ICs have reached its speedlimit is because a large loss is brought about by a capacity of thesubstrate and wires. It is possible to drive in fully high speed withoutincreasing power consumption if an insulator is used for the substrate.From such a reason, an IC having a SOI (semiconductor on insulator)structure has been proposed.

A circuit arrangement of the DRAM is almost the same with that of theaforementioned liquid crystal display device in a case of 1 Tr/cellstructure and in a DRAM having a structure other than that (for example3 Tr/cell structure), no cover film is provided in a memory bit sectionwhen crystallizing an active layer and on the other hand, a cover filmis provided on a region of a driving circuit because it is required tooperate in fully high speed in the same manner with the case of theaforementioned liquid crystal display device to turn the former into lowleak current TFTs and the later into high leak current TFTs.

The basic block structure of such semiconductor memory device is thesame with that shown in FIG. 8A. For example, in the DRAM, the referencenumeral (101) can be a column decoder, (102) a row decoder, (103) amemory elements section, (104) a unit memory bit, (105) bit lines, (106)word lines, and (107) an (insulating) substrate.

A third application example of the present invention is a drivingcircuit for an image sensor or the like. FIG. 8(B) shows an example of aone bit circuit of an image sensor, wherein a flip-flop circuit 108 anda buffer circuit 109 are normally constructed by a CMOS circuit and arerequired to respond in such high speed that they can follow up highspeed pulses applied to a scan line. On the other hand, a TFT 110 in asignal output stage plays a role of a dam that discharges a chargeaccumulated in a capacitor by a photodiode to a data line by receiving asignal from the shift register sections 108 and 109.

Such TFT 110 is required not only to respond in high speed but also tohave less leak current. Accordingly, the region of the circuits 108 and109 is crystallized by providing a cover film to turn it into a highmobility TFT and the region of the TFT 110 on the other hand iscrystallized without providing cover film to turn it into a low leakcurrent TFT in such a circuit.

In the present invention, silicon oxide, silicon nitride or siliconoxinitride (SiN_(x) O_(y)) may be used as the cover film. Although thethicker the cover film, the better the covering ability is, thethickness must be determined considering a mass-producibility andcovering ability thereof because it takes a time to form a thick film.Though the covering ability differs depending on a quality of film,typically the thickness must be more than 20 nm for a silicon oxide filmand more than 10 nm for a silicon nitride film. The thickness is desiredto be 20 to 200 nm for both of the silicon oxide film and the siliconnitride film when considering mass-producibility and reliability.

The above and other advantages of the present invention will become moreapparent in the following description and the accompanying drawings inwhich like reference numerals refer to like parts throughout severalviews.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(C) are top views showing a process of the presentembodiment (crystallization and arrangement of TFT);

FIGS. 2(A-1), 2(A-2) and 2(B) to 2(D) are section views showing theprocess of the embodiment (a selectively crystallizing process)

FIGS. 3(A) to 3(C) are section views showing the process of theembodiment (see a first embodiment);

FIGS. 4(A) to 4(C) are section views showing the process of theembodiment (see the first embodiment);

FIGS. 5(A) to 5(C) are section views showing the process of theembodiment (see a second embodiment);

FIGS. 6(A) to 6(C)are section views showing the process of theembodiment (see a third embodiment);

FIGS. 7(A) to 7(E) are section views showing the process of theembodiment (see a fourth embodiment);

FIG. 8(A) is a block diagram showing a case when the present inventionis applied to an active matrix device;

FIG. 8(B) is a circuit diagram of an exemplified circuit when thepresent invention is applied to a drive circuit of an image sensor;

FIGS. 9(A) to 9(E) are section views showing the process of theembodiment;

FIGS. 10(A) to 10(C) are section views showing the process of theembodiment; and

FIGS. 11(A) to 11(D) are section views showing the process of theembodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[First Embodiment]

A method for fabricating a TFT using a crystal silicon film obtained bycrystallizing an amorphous silicon film using a plurality of islandnickel films formed on Coning 7059 glass substrate as starting pointswill be described in the present embodiment. There are two methods forforming the island nickel films depending on whether it is formed on orunder the amorphous silicon film. FIG. 2(A-1) shows the method whereinit is formed under the film and FIG. 2(A-2) shows the method wherein itis formed on the film. What must be careful especially about the lateris that because nickel is selectively etched after forming it on thewhole surface of the amorphous silicon film in the process, nickel andamorphous silicon react each other and produce nickel silicide, thoughit is a small amount. Because a good crystalline silicon film which thepresent invention aims for cannot be obtained if this nickel silicideremains as it is, it is necessary to remove this nickel silicide fullyby hydrochloric acid or hydrofluoric acid. Due to that, the amorphoussilicon is thinned down from the initial state.

On the other hand, although no such problem is caused in the case of theformer, it is desirable to completely remove the nickel film other thanthat of the island portion 2 by etching also in this case. The influenceof the residual nickel may be suppressed by oxidizing nickel other thanthat of the island region by treating the substrate by oxygen plasma orozone.

In either of the cases, a ground silicon oxide film 1B with a thicknessof 2000 angstrom was formed on a substrate 1A (Coning 7059) by a plasmaCVD method. The amorphous silicon film 1 was fabricated by a plasma CVDmethod or vacuum CVD method with a thickness of 200 to 3000 angstrom orpreferably 500 to 1500 angstrom. The amorphous silicon film was readilycrystallized after removing hydrogen by annealing 0.1 to 2 hours at 350°to 450° C. to keep the hydrogen concentration within the film to lessthan 5 atomic percent.

In the case of FIG. 2(A-1), the nickel film was accumulated up to athickness of 50 to 1000 angstrom or preferably to 100 to 500 angstrom bysputtering and was patterned to form the island nickel regions 2 beforeforming the amorphous silicon film 1.

In the case of FIG. 2(A-2) on the other hand, the nickel film wasaccumulated up to 50 to 1000 angstrom or preferably to 100 to 500angstrom by sputtering and was patterned to form the island nickelregions 2 after forming the amorphous silicon film 1. FIG. 1(A) showsthis state seen from above.

Each of the island nickel is a square with a side of 2 micron and aninterval therebetween was set at 5 to 50 micron or 20 micron forexample. A similar effect may be obtained by using nickel silicideinstead of nickel. A good result could be obtained when the substratewas heated up to 100° to 500° C. or preferably to 180° to 250° C. whenthe nickel was to be formed. It is because an adhesion of the groundsilicon oxide film with the nickel film is improved and because nickelsilicide is produced by the reaction of silicon oxide and nickel. Thesame effect can be obtained by using silicon nitride, silicon carbide orsilicon instead of silicon oxide.

It was then annealed in a nitrogen atmosphere for 8 hours at 450° to580° C. or at 550° C. for example. This annealing may be carried out ina mixed atmosphere of nitrogen and hydrogen. 0r this annealing may becarried out in a hydrogen atmosphere for X₁ hours and then in a nitrogenatmosphere for X₂ hours. FIG. 2(B) shows the intermediate state of thisprocess wherein nickel advance from the island nickel regions 2 near theedge to the center as nickel silicide 3A and portions 3 where nickel hadpassed have become crystal silicon. Then as shown in FIG. 2(C), thecrystallizations which started from the two island nickel films hit andthe nickel silicide 3A remains in the middle, thereby ending thecrystallization.

FIG. 1(B) shows the substrate in this state seen from above, wherein thenickel silicide 3A in FIG. 2(C) is an intercrystalline boundary 4. Whenthe annealing is continued, nickel moves along the intercrystallineboundary 4 and gathers an intermediate region 5 of those island nickelregions (though their original shape is not kept in this state).

Crystal silicon can be obtained by the aforementioned process, but it isnot desirable for nickel to diffuse within the semiconductor coatingfilm from the nickel silicide 3A produced at this time. Accordingly, itis desirable to eliminate the region where nickel is highly concentratedby etching by hydrofluoric acid or hydrochloric acid. By the way,because an etching rate of the nickel and nickel silicide is fullylarge, the silicon film is not affected in the etching by means ofhydrofluoric acid or hydrochloric acid. The regions where the growingpoint of nickel had been located were removed together in the same time.FIG. 2(D) shows the state after the etching. The portion where there wasthe intercrystalline boundary turns out to be a groove 4A. It is notdesirable to form semiconductor regions (active layer or the like) of aTFT so as to pinch this groove. The TFT was arranged so thatsemiconductor regions 6 would not cross the intercrystalline boundary 4as shown in FIG. 1(C). That is, the TFT was formed in a crystal growthregion in the horizontal direction parallel to the substrate, not in thethickness direction of the coating film, by the action of nickel. Then,the growth direction of the crystal could be uniformly arranged andresidual nickel could be minimized. As a result, a high TFTcharacteristics could be obtained. On the other hand, gate wires 7 maycross the intercrystalline boundaries 4.

FIGS. 3 and 4 show examples of the method for fabricating a TFT usingthe crystal silicon obtained in the process described above. In FIG.3(A), the reference character X in the middle indicates the place wherethere was the groove 4A in FIG. 2. As shown in the figure, it wasarranged so that semiconductor regions of the TFT would not cross this Xportion. That is, island semiconductor regions 11a and 11b were formedby patterning the crystal silicon film 3 obtained in the process shownin FIG. 2. Then a silicon oxide film 12 which functions as a gateinsulating film was formed by such methods as RF plasma CVD, ECR plasmaCVD or sputtering.

Further gate electrodes 13a and 13b were formed by forming apolycrystalline silicon film with a thickness of 3000 to 6000 angstromin which 1×10²⁰ to 5×10²⁰ cm⁻³ of phosphorus is doped by a vacuum CVDmethod and then by patterning it (FIG. 3(A)).

Then impurity was doped by a plasma doping method. As a doping gas,phosphine (PH₃) was used for an N type TFT and diborane (B₂ H₆) for a Ptype TFT. The figure shows the N type TFT. An acceleration voltage was80 keV for phosphine and 65 keV for diborane. The impurity was activatedby annealing for four hours at 550° C. to form impurity regions 14athrough 14d. A method of using optical energy such as laser annealing orflash lamp annealing may be also used for the activation (FIG. 3(B)).

Finally, a silicon oxide film with a thickness of 5000 angstrom wasdeposited as an interlayer insulator 15 similarly to a case when TFT isnormally fabricated, and contact holes were formed therethrough to formwires and electrodes 16a through 16d in source and drain regions (FIG.3(C)).

The TFT (N channel type in the figure) was thus fabricated in theprocess described above. The field effect mobility of the TFT obtainedwas 40 to 60 cm² /Vs in the N channel type and 30 to 50 cm² /Vs in the Pchannel type.

FIG. 4 shows a process how a TFT for aluminum gate was fabricated. InFIG. 4(A), the reference character X in the middle indicates the placewhere there was the groove 4A in FIG. 2. As shown in the figure, it wasarranged so that semiconductor regions of the TFT would not cross this Xportion. That is, island semiconductor regions 21a and 21b were formedby patterning the crystal silicon film 3 obtained in the process shownin FIG. 2. Then a silicon oxide film 22 which functions as a gateinsulating film was formed by such methods as RF plasma CVD, ECR plasmaCVD or sputtering. When the plasma CVD method was adopted, a preferableresult could be obtained by using TEOS (tetra-ethoxi-silane) and oxygenas original gases. Then an aluminum film (5000 angstrom thick)containing 1% of silicon was deposited by sputtering and was patternedto form gate wires and electrodes 23a and 23b.

Next, the substrate was soaked into an ethylene glycol solution of 3%tartaric acid and anodic oxidation was implemented by setting platinumas a cathode and an aluminum wire as an anode and by flowing currenttherebetween. The current was applied so that its voltage would increase2 V/min. initially and the voltage was fixed when it reached to 220 V.The current was stopped when it became less than 10 microA/m². As aresult anode oxides 24a and 24b with a thickness of 2000 angstrom wereformed as shown in FIG. 4(A).

Then impurity was doped by a plasma doping method. As a doping gas,phosphine (PH₃) was used for an N type TFT and diborane (B₂ H₆) for a Ptype TFT. The figure shows the N type TFT. An acceleration voltage was80 keV for phosphine and 65 keV for diborane. The impurity was activatedby annealing by laser to form impurity regions 25a through 25d. Thelaser used was a KrF laser (wavelength: 248 nanometer) and five shots oflaser lights having 250 to 300 mJ/cm² of energy density were irradiated(FIG. 4(B)).

Finally, a silicon oxide film with a thickness of 5000 angstrom wasdeposited as an interlayer insulator 26 similarly to the case when TFTis normally fabricated and contact holes were formed therethrough toform wires and electrodes 27a through 27d in source and drain regions(FIG. 4(C)).

The field effect mobility of the TFT obtained was 60 to 120 cm² /Vs inthe N channel type and 50 to 90 cm² /Vs in the P channel type TFT. In ashift register fabricated by using this TFT, operations at 6 MHz in 17volts of drain voltage and at 11 MHz in 20 V were confirmed.

[Second Embodiment]

FIG. 5 shows a case when a TFT for an aluminum gate was fabricatedsimilarly to that shown in FIG. 4. However, the amorphous silicon wasused as an active layer in this embodiment. As shown in FIG. 5(A), aground silicon oxide film 32 was deposited on a substrate 31 and anamorphous silicon film 33 with a thickness of 2000 to 3000 angstrom wasdeposited further on that. An adequate amount of P type or N typeimpurities may be mixed in the amorphous silicon film. Then islandnickel or nickel silicide coating film 34A and 34B were formed asdescribed above and the amorphous silicon film was crystallized bygrowing laterally by annealing for 8 hours at 550° C. or for four hoursat 600° C. in this state.

Then a crystal silicon film thus obtained was patterned as shown in FIG.5(B). At this time, because the silicon film contained a large amount ofnickel in the middle in the figure (the intermediate portion between thenickel or nickel silicide film 34A and 34B), the patterning was carriedout so as to remove such portion and to form island silicon regions 35Aand 35B. Then a substantially intrinsic amorphous silicon film 38 wasdeposited further on that.

After that, as shown in FIG. 5(C), a coating film was formed by suchsubstances as silicon nitride and silicon oxide as a gate insulatingfilm 37. A gate electrode 38 was formed by aluminum and an anodicoxidation was implemented in the same manner with the case of FIG. 4.Then impurity was diffused by an ion doping method to form impurityregions 39A and 39B. Then the TFT was completed by depositing further aninterlayer insulator 40, by forming contact holes and forming metallicelectrodes 41A and 41B at source and drain regions. This TFT ischaracterized in that the semiconductor film at the source and drainportions is thick and that a resistance thereof is small. As a result, aresistance in the source and drain regions is reduced and thecharacteristics of the TFT is improved. Further, contact holes can bereadily formed.

[Third Embodiment]

FIG. 6 shows a process when a CMOS type TFT was fabricated. As shown inFIG. 6(A), a ground silicon oxide film 52 was deposited on a substrate51 and an amorphous silicon film 53 with a thickness of 1000 to 1500angstrom was deposited further on that. Then as described above, islandnickel or nickel silicide coating film 54 was formed and annealing wasimplemented in this state at 550° C. A silicon silicide region 55 movedin the direction of plane, not in the direction of thickness, of thecoating film and the crystallization advances by this process. Theamorphous silicon film changed into crystal silicon as shown in FIG.6(B) by four hours of annealing. The silicon silicide regions 59A and59B were driven away toward the edge along the advancement of thecrystallization.

Then an island silicon region 56 was formed by patterning the crystalsilicon film thus obtained as shown in FIG. 6(B). Here, an attentionmust be paid on that nickel was highly concentrated in the both ends ofthe island region. After forming the island silicon region, a gateinsulating film 57 and gate electrodes 58A and 58B were formed.

Then N type impurity regions 60A and P type impurity regions 60B wereformed by diffusing an impurity by an ion doping method as shown in FIG.8(C). At this time, the doping can be carried out by using phosphorus asa N type impurity (doping gas is phosphine PH₃) and by doping across thewhole surface by 60 to 110 kV of acceleration voltage and then aftercovering the region of the N channel type TFT by a photoresist, by usingboron for example as a P type impurity (doping gas is diborane B₂ H₆)and by doping with 40 to 80 kV of acceleration voltage.

After the doping, the source and drain region were activated byirradiating laser light similarly to the case in FIG. 4. Then the TFTwas completed by depositing further an interlayer insulator 61, byforming contact holes and forming metallic electrodes 62A, 62B and 62Cat the source and drain regions.

[Fourth Embodiment]

FIG. 7 shows the fourth embodiment. The present embodiment relates to amethod in which silicide is produced by reacting a nickel film with aportion of amorphous silicon film by a first heat treatment(pre-annealing) and the amorphous silicon is crystallized by annealingafter removing a non-reactive nickel film.

A ground silicon oxide film 702 (thickness: 2000 angstrom) was formed ona substrate (Coning No. 7059) 701 by a sputtering method. Then a siliconfilm 703 with a thickness of 300 to 800 angstrom or 500 angstrom thickfor example was formed. Further, a silicon oxide film 704 was formed bya plasma CVD method. This silicon oxide film 704 acts as a maskingmaterial and its thickness was preferred to be 500 to 2000 angstrom. Ifit is too thin, the crystallization advances from an unexpected locationby pinholes and if it is too thick, it takes a time to form the film andis not suited for mass-production. Then it was set at 1000 angstromhere.

After that, the silicon oxide film 704 was patterned by a knownphotolithographic process. Then a nickel film 705 (thickness: 500angstrom) was formed by a sputtering method. The thickness of the nickelfilm 705 was preferred to be more than 100 angstrom (FIG. 7(A)).

Then it was annealed for 10 to 60 minutes in a nitrogen atmosphere at250° to 450° C. (a pre-annealing process). For example, it was annealedfor 20 minutes at 450° C. As a result, a nickel silicide layer 706 wasformed within the amorphous silicon. A thickness of this layer wasdetermined by a temperature and time of the pre-annealing and thethickness of the nickel film 705 was almost nothing to do with it (FIG.7(B)).

After that, the nickel film was etched. Nitric or hydrochloric solutionwas suitable for the etching. The nickel silicide layer was barelyetched during the etching of the nickel film by those etchants. In thepresent embodiment, an etchant in which acetic acid was added intonitric acid as a buffer was used. The ratio set was: nitric acid: aceticacid: water =1 :10 :10. After removing the nickel film, it was annealedfor 4 to 8 hours at 550° C. (a crystallizing annealing process).

Several methods were tried in the crystallizing annealing process. Afirst method was to implement this process while remaining the maskingmaterial 704 as shown in FIG. 7(C). The crystallization advances asindicated by arrows in FIG. 7(C). A second method was to anneal afterremoving all the masking material and exposing the silicon film. A thirdmethod was to anneal after removing the masking material and afterforming a new coating film 707 composed of silicon oxide or siliconnitride on the surface of silicon film as a protection film as shown inFIG. 7(D).

Although the first method was simple, the surface of the maskingmaterial 704 reacted with nickel in the pre-annealing step, and becamesilicate in the crystallizing annealing process at a higher temperature,and became hard to be etched. That is, because an etching rate of thesilicon film and masking material 704 becomes almost equal, the portionwhere the silicon film is exposed is also largely etched when themasking material is removed later, creating steps on the substrate.

The second method is very simple and etching can be easily carried outsince the reaction of the masking material with nickel is mild beforethe crystallizing annealing process. However, because the siliconsurface was wholly exposed when the crystallizing annealing was carriedout, characteristics of TFT or the like fabricated later degraded.

Although the third method allowed to firmly obtain a good qualitycrystal silicon film, it was complicated because the number of processeswas increased. As a fourth method which was an improved version of thethird method, a method comprising steps of putting into a furnace in astate when the silicon surface is exposed, heating for about one hour at500° to 550° C. initially in an oxygen flow to form a thin silicon oxidefilm with a thickness of 20 to 60 angstrom thick on the surface andswitching to a nitrogen flow as it is was studied as crystallizingannealing condition. According to this method, an oxide film was formedin the initial stage of the crystallization. And that only the neighborof the nickel silicide layer had been crystallized in this oxidationstage and a region which would be used for the TFT later (right sideportion in the figure) was not crystallized. Due to that, the surface ofthe silicon film at the region far from the nickel silicide layer 706was very flat. The characteristics improved more than that of the secondmethod and was almost equal with that of the third method.

The crystal silicon film was thus obtained. Since then, the silicon film703 was patterned while removing a portion where a value ofconcentration of nickel was high (a region where the origin of growthwas located) and growth points (slanted portions at the end of thearrows in the figure) and while remaining only the region where theconcentration of nickel was low. An island silicon region 708 whichwould be used for an active layer of the TFT was formed as describedabove. Then a gate insulating film 709 composed of silicon oxide with athickness of 1200 angstrom was formed covering the region 708 by aplasma CVD method. Further, a gate electrode 710 and a wire 711 in afirst layer were formed by a phosphorus doped silicon film with athickness of 6000 angstrom and source/drain regions 712 were formed byinjecting an impurity into the active layer 708 in a self-aligningmanner using the gate electrode 710 as a mask. It was then effective forimproving the crystallinity to irradiate visible or near infrared stronglight. Further, a silicon oxide film with a thickness of 6000 angstromwas formed by a plasma CVD method as an interlayer insulator 713.Finally, contact holes were created in this interlayer insulator and awire 714 in a second layer, source/drain electrode and wires 715 wereformed by an aluminum film with a thickness of 6000 angstrom. The TFTwas completed by the process described above (FIG. 7(E)).

[Fifth Embodiment]

FIG. 9 shows the present embodiment. In the present embodiment, apolysilicon TFT is formed in a peripheral circuit and an active matrixregion of a TFT type liquid crystal display device.

At first, a ground oxide film 121 was deposited into a thickness of 20to 200 nm by a sputtering method on a glass substrate 120 having a heatresisting quality such as a silica glass. Further on that, an amorphoussilicon film was deposited into a thickness of 30 to 50 nm by a plasmaCVD method or vacuum CVD method using mono-silane or di-silane as anoriginal material. Here, a concentration of oxygen or nitrogen in theamorphous silicon film should have been less than 10¹⁸ cm⁻² orpreferably less than 10¹⁷ cm⁻². The oxygen concentration was set to beless than 10¹⁷ cm⁻² in the present embodiment. A silicon oxide film witha thickness of 100 to 150 nm or silicon nitride film with a thickness of30 to 100 nm was formed on the amorphous silicon film by a sputteringmethod as a cover film. It was then patterned to leave a cover film 122only in the peripheral circuit region. Then it was crystallized byleaving for 4 to 100 hours in an argon or nitrogen atmosphere (600° C.)containing 20 to 100 volume percent of oxygen or hydrogen. As a result,a crystallinity of a silicon film 123A in the peripheral circuit regionwas good and that of a silicon film 123B in the picture element regionwas not good. FIG. 9(A) shows this state.

Next, the silicon film was patterned into a shape of island to form aperipheral circuit TFT region 124A and a picture element TFT region 124Bas shown in FIG. 9(B). Then a gate oxide film 125 was formed by means ofsputtering or the like. It can be formed by a plasma CVD method usingTEOS (tetraethoxisilane) instead of the sputtering method. It isdesirable to anneal for 0.5 to 3 hours at a temperature more than 650°C. during or after the formation when forming the film using TEOS.

After that, a N-type silicon film with a thickness of 200 nm to 2 micronwas formed by a LPCVD method and by patterning it, gate electrodes 128Athrough 128C were formed on each island region. A metallic materialhaving a relatively good heat resistance such as tantalum, chrome,titanium, tungsten and molybdenum may be used instead of the N-typesilicon film.

Then, an impurity was injected to the island silicon film of each TFT byan ion doping method in a self-aligning manner using the gate electrodesection as a mask. At this time, phosphorus was injected across thewhole surface employing phosphine (PH₃) as a doping gas at first andafter covering the right side of the island region 124A and the matrixregion in the figure by a photoresist, boron was injected to the leftside of the island region 124A employing diborane (B₂ H₆) as a dopinggas. The dosage of phosphorus was set to be 2 to 8×10¹⁵ cm⁻² and that ofboron was 4 to 10×10¹⁵ cm⁻² so that the dosage of boron would exceedthat of phosphorus. A P-type region 127A and N-type regions 127B and127C were thus created.

It was activated by annealing for 2 to 24 hours at a temperature between550° and 750° C. The thermal annealing was carried out for 24 hours at600° C. in the present embodiment. This annealing process activated theregion in which ions were injected.

This process can be implemented by laser annealing. Because a thermaldamage on the substrate is small when annealed by laser, a normalnon-alkaline glass such as Coning 7059 can be used. Further, at thattime, a material having an inferior heat resistance such as aluminum canbe used as a material for the gate electrode. The P-type region 127A andN-type regions 127B and 127C were created by the process describedabove. A sheet resistance of those regions was 200 to 800 ohm/sheet.

After that, a silicon oxide film with a thickness of 300 to 1000 nm wasformed across the whole surface by a sputtering method as an interlayerinsulator 128. This may be a silicon Oxide film formed by a plasma CVDmethod. A silicon oxide film having a good step coverage can be obtainedby the plasma CVD method especially using TEOS as an original material.

Then an ITO film was created by a sputtering method and was patterned toform a picture element electrode 129. Contact holes were created insource/drain (impurity regions) of the TFT to form wires 130A through130E made of chrome or titanium nitride. FIG. 9(C) shows that aninverter circuit have been created by the NTFT and PTFT on the leftside. The wires 130A through 130E may be a multi-layered wire withaluminum based on chrome or titanium nitride in order to reduce a sheetresistance. Finally, it was annealed for 0.5 to 2 hours at a temperaturebetween 200° and 350° C. in hydrogen to reduce a dangling bond of thesilicon active layer. The peripheral circuit and the active matrixcircuit could be integrally created. In the present embodiment, atypical mobility was 80 cm² /Vs in the NMOS, 50 cm² /Vs in the PMOS inthe peripheral circuit section and 5 to 30 cm² /Vs in the pictureelement TFT (NMOS).

[Sixth Embodiment]

FIG. 10 shows the present embodiment. In the present embodiment, adifference of mobility of NMOS and PMOS is reduced in a CMOS circuitutilizing the present invention. At first, a ground oxide film 132 wasdeposited into a thickness of 20 to 200 nm by a sputtering method on aConing 7059 substrate 131. Further on that, an amorphous silicon filmwas deposited into a thickness of 50 to 250 nm by a plasma CVD method orvacuum CVD method using mono-silane or di-silane as an originalmaterial. Here, a concentration of oxygen or nitrogen in the amorphoussilicon film should have been less than 10¹⁸ cm⁻² or preferably lessthan 10¹⁷ cm⁻². The vacuum CVD method was suited for this purpose. Theoxygen concentration was set to be less than 10¹⁷ cm⁻² in the presentembodiment.

A cover film 133 (a silicon oxide film with a thickness of 50 to 150 nm)was provided on the region of PMOS. Then it was crystallized byannealing for 4 to 100 hours at 600° C. in an argon or nitrogenatmosphere at 600° C. containing more than 50 percent of oxygen orhydrogen. As a result, although a crystallinity of a region 134A underthe cover film was good, that of a region 134B where there was no coverfilm was not good. FIG. 10(A) shows this state.

Next, the silicon film was patterned into a shape of island to form aPMOS region 135A and an NMOS region 135B as shown in FIG. 10(B). Then asilicon oxide film 125 with a thickness of 50 to 150 nm was formed by asputtering method covering those island regions as a gate insulatingfilm 138. Then an aluminum film was formed with a thickness of 200 nm to2 micron by a sputtering method, and patterned to form a gate electrode.An anodic oxide film was formed on the upper and side surfaces of thegate electrode by feeding power to it in an electrolyte. Gate electrodesections 137A and 137B were formed on each island region by the processdescribed above.

Then, an impurity was injected to the island silicon film of each TFT byan ion doping method in a self-aligning manner using the gate electrodesection as a mask. At this time, phosphorus was injected across thewhole surface employing phosphine (PH₃) as a doping gas at first andafter covering only the island region 135B in the figure by aphotoresist, boron was injected to the island region 135A employingdiborane (B₂ H₆) as a doping gas. The dosage of phosphorus was set to be2 to 8×10¹⁵ cm⁻² and that of boron to be 4 to 10×10¹⁵ cm⁻² so that thedosage of boron would exceed that of phosphorus.

Although the crystallinity of the silicon film is broken by the dopingprocess, it was possible to keep its sheet resistance around 1kohm/sheet. However, if the sheet resistance of this degree is to much,the sheet resistance can be lowered by annealing further for 2 to 24hours at 600° C. The same effect can be obtained by irradiating such astrong light as laser light.

A P-type region 188A and N-type regions 138B were thus created. A sheetresistance of those regions was 200 to 800 ohm/sheet. Then a siliconoxide film with a thickness of 300 to 1000 nm was formed across thewhole surface by a sputtering method as an interlayer insulator 139.This may be a silicon oxide film formed by a plasma CVD method. Asilicon oxide film having a good step coverage can be obtained by theplasma CVD method especially using TEOS as an original material.

Then contact holes were created in source/drain (impurity regions) ofthe TFT to form aluminum wires 140A through 140D. Finally, it wasannealed for 2 hours at a temperature between 250 and 350° C. inhydrogen to reduce a dangling bond of the silicon film. A typicalmobility of the TFT obtained by the process above was 60 cm² /Vs both inthe PMOS and NMOS. When a shift resistor was fabricated using theprocess of the present embodiment, an operation of more than 10 MHz with20 V of drain voltage was confirmed.

[Seventh Embodiment]

FIG. 11 shows the present embodiment. The present embodiment relates toa circuit in which a transistor and silicon resistance are combined.Silicon doped by impurity can be used as a protecting circuit of thetransistor. At first, a ground oxide film 141 was deposited into athickness of 20 to 200 nm by a sputtering method on a Coning 7059substrate 140. Further on that, an amorphous silicon film 142 wasdeposited into a thickness of 100 to 250 nm by a plasma CVD method orvacuum CVD method using mono-silane or di-silane as an originalmaterial. Here, a concentration of oxygen or nitrogen in the amorphoussilicon film should have been less than 10¹⁸ cm⁻² or preferably lessthan 10¹⁷ cm⁻².

A cover film 143 of a silicon oxide film with a thickness of 20 to 200nm was deposited and it was crystallized by annealing for 4 to 100 hoursin an argon or nitrogen atmosphere at 600° C. FIG. 11(A) shows thisstate.

Next, the silicon film was patterned into a shape of island to form atransistor region 144A and a resistance region 144B as shown in FIG.11(B). Then a silicon oxide film with a thickness of 50 to 150 nm wasformed by a sputtering method covering those island regions as a gateinsulating film 145. Then an aluminum film was formed with a thicknessof 200 nm to 2 micron by a sputtering method, and patterned to form agate electrode. An anode oxide film was formed on the upper and sidesurfaces of the gate electrode by feeding power to it in an electrolyte.Gate electrode section 146 was formed on each island region by theprocess described above.

Then, an impurity, e.g. phosphorus, was injected to the island siliconfilm of each TFT by an ion doping method in a self-aligning manner usingthe gate electrode section as a mask. The dosage of phosphorus was 2 to8×10¹⁵ cm⁻².

Impurity regions 147A and 147B were created by the doping processdescribed above. Since the same amount of impurity is being injected inthose two impurity regions, they show the same resistivity when they arethermally annealed as they are. However, there is a such case forexample when a higher resistance is demanded to the latter whereas alower resistance is always demanded to the former. Then, a cover film148 of a silicon oxide film with a thickness of 50 to 150 nm was formedonly on the transistor region as shown in FIG. 11(C). It was thenannealed for 4 to 20 hours at a temperature between 550° and 650° in anargon or nitrogen atmosphere containing more than 50 volume percent ofoxygen or hydrogen. Phosphine (PH₃) may be used instead of oxygen orhydrogen. However, the annealing temperature is preferable to be lessthan 800° C. because if the annealing temperature is too high, phosphineis thermally decomposed and diffuses within the semiconductor, loweringthe resistivity on the contrary. Diborane (B₂ H₆) may be used when theimpurity region of the resistance is P-type.

While a sheet resistance of the impurity region of the transistor was 20to 800 ohm/sheet, that of the impurity region of the resistance was 2 kto 100 kohm/sheet by the process above. Then a silicon oxide film with athickness of 300 to 1000 nm was formed across the whole surface by asputtering method as an interlayer insulator 149. This may be a siliconoxide film formed by a plasma CVD method. A silicon oxide film having agood step coverage can be obtained by the plasma CVD method especiallyusing TEOS as an original material.

Then contact holes were created in source/drain (impurity regions) ofthe TFT to form aluminum wires 150A through 150C. Finally, it wasannealed for 0.5 to 2 hours at a temperature between 250° to 350° C. inhydrogen to reduce a dangling bond of the silicon film. A sheetresistance of the regions which had the same thickness and to which thesame amount of impurity was injected could be differentiated by theprocess described above.

As described above, the present invention is an epoch-making inventionin a sense that it promotes the implementation of lower temperature andshorter time crystallization of amorphous silicon and provides animmeasurable benefit to the industry because facility, equipment andtechnique therefor are very common and are excellent formass-production. Although the explanation has been made centering onnickel in the aforementioned embodiments, the same process can beapplied to another metal element that accelerates the crystallization,i.e. any one of Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Sc, Ti, V, Cr, Mn, Cu,Zn, Au and Ag.

For example, assuming that it takes two minutes to treat one sheet ofsubstrate, while 15 annealing furnaces were necessary in theconventional solid phase growing method because at least 24 hours ofannealing was necessary, the present invention allows to reduce thenumber of the annealing furnaces to less than 1/6 of that because theannealing time can be shortened to four hours or less. The improvementof productivity and the reduction of amount of investment on facilitybrought about by that will lead to the drop of substrate processing costas well as to the drop of a cost of TFTs and thereby to the rise of newdemand. Accordingly, the present invention is very beneficial to theindustry and deserves to be patented.

Further, the present invention solves the problem in the conventionalfabrication process of crystalline silicon TFTs by such minimummodification of the crystallization condition of active layer of theTFTs that whether a cover film exists or not.

The present invention allowed to improve especially a reliability andperformance of a dynamic circuit and of a device having such circuit.Conventionally, although crystalline silicon TFTs had low an ON/OFFratio for such purpose as an active matrix of a liquid crystal displayand was difficult in various ways to put it into practical use, suchproblems have been considered to be solved by the present invention.Although not shown in the embodiments, it will be apparent that thepresent invention will be effective, when implemented, in TFTs used asmeans for implementing a stereo-monocrystal semiconductor integratedcircuit.

For example, a memory elements section can be constructed byconstructing a peripheral logic circuit by semiconductor circuits On amonocrystal semiconductor and by providing TFTs on that through theintermediary of an interlayer insulator. In this case, the memoryelements section can be a DRAM circuit using the TFTs of the presentinvention and its driving circuit is constructed by beingCMOS-implemented to the monocrystal semiconductor circuit. Furthermore,when such circuit is used for a microprocessor, its area can be savedbecause the memory section is raised to the upstairs. Thus the presentinvention is considered to be a very beneficial invention to theindustry.

While the present invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that the foregoing and other changes in formand details can be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A semiconductor device formed on an insulatingsurface, said device comprising:a semiconductor island comprisingcrystalline silicon; source, drain and channel regions formed withinsaid semiconductor island; a gate electrode adjacent to saidsemiconductor island with a gate insulating film interposedtherebetween, wherein said semiconductor island contains a materialselected from the group consisting of nickel, iron, cobalt, ruthenium,rhodium, palladium, osmium, iridium, platinum, scandium, titanium,vanadium, chromium, manganese, copper, zinc, gold and silver at asubstantially uniform concentration throughout said semiconductorisland.
 2. The semiconductor device of claim 1 wherein said material iscontained at a concentration not higher than 1 atomic %.
 3. Thesemiconductor device of claim 1 wherein said crystalline silicon furthercontains hydrogen at 5 atomic % or less.
 4. A semiconductor deviceformed on an insulating surface, said device comprising:a semiconductorisland comprising crystalline silicon; and a gate electrode adjacent tosaid semiconductor island with a gate insulating layer interposedtherebetween, wherein said semiconductor island contains a materialcapable of promoting a crystallization of an amorphous silicon at asubstantially uniform concentration throughout said semiconductorisland, and hydrogen at a concentration of 5 atomic % or less.
 5. Thesemiconductor device of claim 4 wherein said material is a metalselected from the group consisting of nickel, iron, cobalt, ruthenium,rhodium, palladium, osmium, iridium, platinum, scandium, titanium,vanadium, chromium, manganese, copper, zinc, gold and silver.
 6. Asemiconductor device formed on an insulating surface, said devicecomprising:a semiconductor island comprising crystalline silicon dopedwith hydrogen at 5 atomic % or less; and a gate electrode adjacent tosaid semiconductor island with a gate insulating layer interposedtherebetween, wherein said semiconductor island contains a materialselected from the group consisting of nickel, iron, cobalt, ruthenium,rhodium, palladium, osmium, iridium, platinum, scandium, titanium,vanadium, chromium, manganese, copper, zinc, gold and silver at asubstantially uniform concentration throughout said semiconductorisland.